The present invention relates in general to fasteners, and, more in particular, to fasteners that lock when set and that develop a predetermined clamp-up load while being set.
Nut- and bolt-type fasteners go back a long time. They secure workpieces or sheets together by applying a clamp-up force between a head of the bolt and the nut, with this load carried by a threaded engagement between the two. Wrenching surfaces of the nut and bolt accept wrenches that tightly join the fasteners and sheets together. Broadly, another name for a bolt is a "pin" or "shear pin" and another name for a nut is a "collar."
Many environments using fasteners require that the fasteners have extremely high integrity and strength. A fastener must bear loads not only along its longitudinal axis but radially of that axis. When fasteners join two or more sheets together and the sheets are loaded in their planes with different loads, or loads acting in different directions, the sheets tend to slide over each other. Fasteners in the sheets become loaded in shear by their resistance to this type of loading.
Although a fastener may indefinitely sustain constant load, when the load cycles, the possibility of fatigue failure arises. A fastener bearing a high clamp-up load resists fatigue failure better than one with a low load.
An obviously desirable feature of a fastener is that it does not come apart in service. Various devices have been used to keep a collar and pin together. One way of locking a collar and a pin together is to deform the threads of the collar so that they bear in radial compression against the threads of the pin. With this lock, resistance to unthreading is purely frictional. Threads are commonly deformed at the factory, in preference to the field, but field deformation has also been practiced. With threads deformed at the factory, special coatings to resist corrosion or for lubrication are removed during the threading of the collar onto the pin by the friction of the threads. Thread deformation in the field requires a fastener of special configuration to permit such deformation and an additional installation step.
It is also highly desirable to know just what clamp-up load the fastener applies to a structure. Clamp-up load correlates to the resistance of a collar to further threading onto a pin. As clamp-up force increases, resistance to further threading increases and the torque required to turn the collar increases. This fact has been used in fasteners to develop a predetermined clamp-up load. In one prior art fastener, a wrenching section connects to a collar by a frangible breakneck that breaks upon the application of a predetermined torque corresponding to the desired clamp-up load.
A problem with a fastener having a frangible breakneck is that it generates a waste piece during installation that requires attention in its removal. The existence of the waste piece in some environments would be extremely hazardous, for example, in fuel tanks of aircraft.
The features of a thread lock and a frangible breakneck for clamp-up load control have been combined in one collar. The combination has had its shortcomings. Factory deformed threads of the collar effect the thread lock. As such, the collar does not freely thread onto the pin and resists threading. This makes setting somewhat more difficult and compromises protective and lubricant coatings. Clamp-up load control is through a wrenching section that separates from a threaded section by a frangible breakneck that fails at a predetermined load. This creates an excess piece that must be removed. This type of fastener is also comparatively difficult to make because it requires considerable machining, and therefore the fastener is expensive. For accurate control of clamp-up load, the breakneck must be made to very close dimensional tolerances. Tolerance control is made difficult by machine tool wear and because the breakneck becomes elliptical after the thread-locking feature has been incorporated. Alternate methods of forming the frangible breakneck, such as roll forming, are not available because the part is hollow. The frictional drag between the pin and the collar in a fastener system employing a preexisting deformed thread lock results in a broad range in clamp-up force because the drag varies between large limits and is an important component in the resistance that effects failure of the frangible breakneck. Further, with the frangible section, a circular band of material in the zone of failure is created. Where corrosion control is important, this circular band cannot be protected by corrosion inhibitors at the time of fastener manufacture.
A second approach to a locking system employs a pin having a groove for receipt of deformed collar material. The collar is threaded onto the pin to develop desired clamp-up load, and is then deformed radially inward into the groove so that the deformed collar material is restrained by the walls of the groove and establishes interference. The groove can be made axial or annular. In one type of such a fastener, a collar is threaded onto a pin with one setting tool. A second setting tool radially deforms the collar into threads of the pin to effect the interference lock.
In the parent of this application a unique collar and pin are disclosed. The collar will be described in greater detail subsequently, but for purposes here it has a plurality of lobes spaced along its outer surface. A setting tool applies a compressive, radially inwardly directed load on these lobes and in conjunction with the resistance of sheets fails the lobes in radial compression at a predetermined load. Upon failure, collar material inwardly of the lobes moves into the collar's axial bore and into locking engagement with the pin. The pin has a generally hexagonal configuration in the zone where locking occurs. The flats of the hexagon provide volumes for collar material and interference between the collar and the pin. As a result, when collar material displaces into the volumes confronting the flats, the collar and pin lock together.
It is highly desirable in any fastener that employs a threaded coupling to have a large area of the flanks of the threads in engagement. If with the arrangement just discussed the flats are too extensive, too little flank area will be available for engagement with the threads of the collar.
It is also highly desirable to maintain the minimum radius to the flats at a diameter at least as great as the root diameter of the threads so that the tensile strength of the pin is not affected by the provisions for displaced collar material.
In the disclosed fastener, three lobes cooperate with six flats of the pin to effect a lock. This means, in the usual case, that there will be three zones of collar material displaced into the three volumes contiguous with three flats, the three other volumes being unaffected. Resistance to rotation will be determined by the depth of the flats from a circle of a radius to the edges of each flat. Resistance will also depend on the shape of the flat. If the material in the volumes can readily ramp up the flat to flow into an adjoining void volume, the resistance to unthreading decreases substantially.